How do I treat a goldfish pond green with algae? Visibility is ~2 inches. It's not the wall coating I'm concerned about, it's the water itself. I know how I'd treat it indoors - with a scraper and a water change, but that won't work in a pond. (Well, I suppose a water change would help...) Currently, I've replaced the bacterial sponge in the pump with cotton batting for a finer filtering.